1. The Field of the Invention
The present invention relates to methods and apparatus for displaying images, and more particularly, to methods and apparatus for increasing the perceived resolution of the displayed images and compressing image data to enable control signals to be efficiently transmitted to display devices.
2. The Prior State of the Art
Color display devices have become the principal display devices of choice for most computer users. The display of color on a monitor is normally achieved by operating the display device to emit light, typically a combination of red, green, and blue light, which results in one or more colors being perceived by a human viewer.
In cathode ray tube (CRT) display devices, the different colors of light are generated by phosphor coatings that may be applied as dots in a sequence on the screen of the CRT. A different phosphor coating is normally used to generate each of the red, green, and blue colors, resulting in repeating patterns of phosphor dots. When excited by a beam of electrons, the phosphor dots generate the colors red, green and blue.
The term pixel is commonly used to refer to one spot in, for example, a rectangular grid of thousands of such spots. Many computer applications and other types of applications assume that each pixel corresponds to a square portion of a display screen. Pixels are individually used by a computer to form an image on the display device. For a color CRT, where a single triad of red, green and blue phosphor dots cannot be addressed, the smallest possible pixel size will depend on the focus, alignment and bandwidth of the electron guns used to excite the phosphors. The light emitted from one or more triads of red, green and blue phosphor dots, in various arrangements known for CRT displays, tends to blend together giving, at a distance, the appearance of a single colored light source representing a pixel.
Liquid crystal displays (LCDs) and other flat panel display devices are commonly used in portable computer devices in place of CRTs. This is because flat panel displays tend to be small and lightweight in comparison to CRT displays. In addition, flat panel displays generally consume less power than comparably sized CRT displays, making them better suited for battery powered applications. As the quality of flat panel color display devices increases and their cost decreases, flat panel displays continue to replace CRT displays in desktop applications. Accordingly, flat panel displays, and LCDs in particular, are becoming ever more common.
Color LCD displays are exemplary of display devices that utilize multiple separately addressable and controllable elements, referred to herein as xe2x80x9cpixel sub-components,xe2x80x9d to represent each pixel of an image being displayed. In many known LCD displays, each pixel is a single square element that includes non-square red, green and blue (RGB) pixel sub-components. When combined, the RGB pixel sub-components form the square pixel.
FIG. 1 illustrates a portion of a known LCD device 100. The illustrated LCD device 100 includes four columns (C1-C4) and three rows (R1-R3) of pixels, each of which has a separate red pixel sub-component 102, green pixel sub-component 104 and blue pixel sub-component 106. Each of the three pixel sub-components 102, 104, 106 is three times taller than it is wide. As a result of their aspect ratios of 3:1, the RGB pixel sub-components 102, 104, 106 produce a square pixel. The RGB pixel sub-components 102, 104, 106 are arranged to form stripes along LCD device. The RGB stripes normally run the entire length of the display in one direction. Common LCD devices used for computer applications are wider than they are tall and tend to have RGB stripes running in the vertical direction. For convenience, the invention is described herein primarily in the context of LCD devices having vertical stripes, although the principles of the invention apply to display devices having other pixel sub-component configurations.
In color displays, the intensity of the emitted red, green and blue light produced by the corresponding pixel sub-components 102, 104, 106 can be varied to generate the appearance of almost any desired color pixel. Emitting no light from the pixel sub-components 102, 104, 106 produces a black pixel, whereas emitting all three colors at 100 percent intensity results in a white pixel.
While conventional displays have proved satisfactory for many applications, there is a need for resolution improvement. The resolution of flat panel display devices, which is considerably lower than the resolution achieved by print media, makes it difficult to display high quality Latin-based and similar alphanumeric characters at small text sizes commonly used for reading. The problem of low resolution is even more pronounced when complex script languages, such as Japanese, Chinese, Korean, and the Indic languages, are displayed. Ideographic languages, such as Japanese, use large numbers of Kanji characters or other characters that often rely as heavily on vertical resolution as horizontal resolution.
The most complex Kanji character has nine horizontal lines, thus requiring 17 pixels to represent the lines and the spaces between them. At current display resolutions near 100 dots per inch, a true representation is not feasible at font sizes smaller than about 14 point type ({fraction (14/72)} of an inch). At 100 dots per inch, display devices simply do not have enough dots to depict complex Kanji characters at text sizes that would be preferred for comfortable reading.
Japanese books are commonly printed in 9, 10 and 11-point type, which are similar to those used in Western books. This is a desirable size for reading based on human physiology. Manga comic books, hugely popular in Japan, use even smaller type sizes. Further complicating matters is the fact that small frutigana characters used to provide Japanese with pronunciation guidance for less-common Kanji characters are typically displayed using 3 or 4 point type. Representing characters at these sizes on computer screens, particularly LCDs, presents huge challenges.
One known technique to addressing the unavailability of screen pixels to represent the full strokes of complex characters has been to use hand-tuned bitmaps at small sizes. Unfortunately, these hand-tuned bitmaps are, at best, crude representations of characters that cannot be drawn accurately at the desired display sizes given the resolution of conventional displays. In such implementations, some strokes in the true character outlines have to be run together or completely eliminated. Decisions as to which strokes can be edited in such a manner require extensive knowledge of the specific language and involve a great deal of time and effort. For example, it would not be unusual for it to take over two years to produce a single typeface in this manner, because there are upwards of 7,000 characters involved in some languages. Embedded bitmap fonts also have the disadvantage of requiring large amounts of memory to store. Because of such limitations, Japanese operating systems tend to ship with very few supported typefaces. In fact, one common operating system of Microsoft Corporation of Redmond, Washington, for Japanese personal computers currently includes only two Japanese typefaces, MS-Gothic and MS-Mincho. Although Kanji characters represent a particularly difficult type to render on LCD display devices, similar low resolution problems are encountered when displaying any characters.
In view of the foregoing, it is apparent that there is a need for improved techniques for displaying images on display devices. It would be desirable for any such techniques to improve resolution in at least one, and more preferably, two-dimensions (i.e., the horizontal and vertical dimensions). It would also be desirable, from the manufacturing standpoint, for at least some new display devices to be manufactured using existing display technology and manufacturing equipment, thereby avoiding the expense that would be associated with developing or obtaining new display device manufacturing equipment.
The present invention relates to methods and systems for improving the resolution of displayed images in the horizontal and vertical dimensions of LCD or other flat panel display devices that have separately controllable pixel sub-components. One factor that is responsible for at least some of the improved resolution is that the separately controllable pixel sub-components, rather than full pixels, are treated as individual luminous intensity sources. Each pixel sub-component represents a spatially different portion of the image. In order to obtain such results, spatially different sets of one or more samples of the image data are mapped to the individual pixel sub-components, rather than to entire pixels.
Such displaced sampling is responsible for increasing the resolution of the display device in the direction perpendicular to the stripes of the display device. Increased resolution in the orthogonal direction (i.e., the direction parallel to the stripes) is achieved by increasing the pixel sub-component density beyond that of conventional display devices. For instance, each region of the display device that would ordinarily consist of a single pixel with three pixel sub-components is configured to include two or three full pixels, each having three pixel sub-components. The pixel sub-components have heights 1.5 times greater than their widths if the pixel sub-component density is doubled, or are square if the density is tripled. The pixel sub-component density can be increased by other factors, as well, although a factor of two or three has the advantage that the height dimension is no smaller than the width dimension, and existing pixel sub-component manufacturing techniques can be readily adapted to construct such display devices.
Display devices having the foregoing pixel and pixel sub-component configurations can enable images to be displayed with resolutions that are improved both in the vertical and horizontal dimensions compared with conventional rendering processes. The two-dimensional improvement in resolution can be particularly advantageous for displaying complex characters, such as Kanji characters, that rely heavily character features that have fine detail in both the horizontal and vertical dimensions.
Many existing computers do not have the capability of transmitting luminous intensity values in control signals to display devices at a rate great enough to support the increased pixel sub-component densities of the display devices disclosed herein. In order to make use of the available bandwidth of such computers, the image data processing and image rendering processes of the invention also extend to image data compression techniques.
The image data compression processes are adapted to encode the luminous intensity values to be applied to a set of vertically adjacent pixels referred to as a control element of the display device. The control element includes a set of two vertically adjacent pixels when the pixel sub-component density is doubled and a set of three vertically adjacent pixels when the pixel sub-component density is tripled, such that the control element occupies a substantially square portion of the display device.
The luminous intensity values applied to the pixel sub-components in a control element are encoded in a data structure having a length, for example, of 8, 16, or 24 bits. The data structure includes a red luminous intensity value, a green luminous intensity value, a blue luminous intensity value, and a bias value. The red, green, and blue luminous intensity values correspond to the overall or average luminance to be generated in the pixel sub-components of the control element. The bias value indicates the relative luminance between the multiple pixels in the control element. For instance, if the control element includes two vertically adjacent pixels, the bias value indicates whether the luminance is to be biased toward the upper pixel, toward the lower pixel, or to be distributed evenly.
The data compression techniques of the invention allow the control signal to be transmitted to the display device at substantially the same rate as would be experienced if the pixel sub-component density were not increased. In other words, if a particular display system operating on a computer transmits 16 bits of data per square pixel in the absence of increased pixel sub-component density, the compressed control signal for the display device having the increased pixel sub-component density can also use 1.6 bits of data per control element (i.e., square region of the display device). Of course, the cost of the data compression is generally the loss of some resolution compared to the resolution that would be obtained if each pixel were to be independently controlled without data compression.
The invention also extends to display devices that are further adapted to decrease the color artifacts that can be generated from treating each pixel sub-component as a separate luminance source. In one implementation, the position of the red and blue pixel sub-components in a pixel is transposed in alternating adjacent rows. This pixel sub-component configuration breaks; up the vertical stripes of same colored red and blue pixel sub-components that are present in many conventional display devices, thereby diminishing the color fringing effects that can be experienced. In other implementations, successive rows of pixels have red, green, and blue pixel sub-components that are offset by ⅓ or ⅔ the width of the full pixel, so that the stripes are not formed from same-colored pixel sub-components, but are instead formed from alternating red, green, and blue pixel sub-components.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.